301
longer-term measurements – implications of application in the field
302
relevance of the membrane module in terms of an actual application in the field
307
that attempts to improve the quality of the biogas, an adequate degree of process
308
durability should be acquired. Therefore, performance of the PI membrane
309
module was further analyzed over the longer-term by running permeation
310
experiments with real biogas (generated by an anaerobic digestion plant located
311
in the countryside of Hungary). Furthermore, implementation of the whole test rig
312
in an industrial setting is accompanied with the advantage of a continuous gas
313
supply and the availability of sufficient feed volumes, which would otherwise limit
314
the exploitation of permeation capacities over a more extensive period of time.
315
1
characterised as a clearly distinguishable quality compared to the one applied
317
during laboratory tests (Table 2). This might be attributed to differences in the
318
attributes of biotic and abiotic processes, i.e. in terms of the (i) composition of
319
underlying microbial consortia, (ii) source and complexity of the feedstock to be
320
utilized, (iii) operational settings of the fermenters, etc. During the permeation
321
stability tests, separation conditions were constants (Table 3) for almost 9 hours
322
during the experiment (Figs. 6 and 7). It should be noted that besides the clearly
323
identifiable components, namely CH4, CO2 and N2, the raw biogas, on average,
324
contains a comparable amount of trace substances to the biogas evolved in the
325
laboratory-scale bioreactor (Table 2). However, the similarities regarding the
326
distribution (partial concentrations) of these components remain unknown and
327
such an analysis could be a subject of a future study to elaborate on such related
328
effects. Actually, based on the already published experiences in the existing
329
literature, pro-longed operation of the biogas-upgrading membrane permeation
330
system can require the pretreatment of raw fermenter off-gas to get rid of
331
particular secondary components (i.e. ammonia, hydrogen sulfide and water
332
vapor that may damage the membrane material over time) by drying,
333
condesnation and desulphurization before conveying the biogas to the
334
membrane purification technology (Miltner et al., 2010, 2009). Such an action
335
can help to extend membrane lifetime and preserve its performance (Stern et al.,
336
changes in the compositions were recorded and, therefore, the purification
340
performance could be considered quite stable throughout the test period.
341
Similarly to the results of the other gas mixtures discussed above, a considerable
342
degree of CH4/CO2 separation was achieved. However, the removal of nitrogen
343
1
gas seemed to be challenging, in accordance with statements made in Section 5.
344
Under the circumstances mentioned in Table 3, a reasonable and steady level of
345
CH4 recovery (Ymethane > 82 %) was accomplished with a corresponding methane
346
concentration of 81-82 vol.% in the retentate. Overall, these research outcomes
347
imply that the gas permeation process was able to function properly over an
348
extended period of time without considerable variation in the separation
349
efficiency. Thus, it can be deduced that the PI membrane employed may be a
350
worthy candidate for further investigation and possible installation at biogas
351
plants. However, the experiments conducted point to the fact that this particular
352
module should be applied as one component of a multi-stage (sequential)
353
membrane system, enriching the CH4 content of the biogas to the desired level of
354
biomethane quality (Makaruk et al., 2010). Such a system is supposed to
355
manage the efficient separation of N2 from CH4 and attain large Ymethane values to
356
reduce losses in the permeate (increase product recovery) (Rautenbach and
357
Welsch, 1993) and consequently, minimise the environmental impacts
358
associated with the emission of methane. Many times, however, high methane
359
purities may be attained only with compromises in methane recovery, when
360
some methane is lost in the permeate (Sun et al., 2015). Under these conditions,
361
for instance, the permeate with methane content can be recycled and burnt in
362
gas engines at the biogas plant (Miltner et al., 2009).
363 364
7. Conclusions
365 366
In this paper, a polyimide gas separation membrane was investigated in
367
terms of biogas purification. The results showed that the
feed-to-permeate-368
pressure ratio as well as the splitting factor had a notable effect on the
369
performance of the process. In fact, under actual operating circumstances, the
370
1
conditions and accordingly, could be as high as 11-12 in some cases. However,
374
primarily due to the insufficient CH4/N2 separation capacity of the membrane, it
375
was not possible to upgrade the real biogas in the same manner and additional
376
research into the subject is encouraged. Nevertheless, tests revealed an
377
adequate level of endurance of the membrane permeation process over the
378
longer-term, leading to the conclusion that the process, based on the module that
379
contains PI hollow fibers, is worthy of further elaboration under industrial
380
conditions in the field. The appropriate design of the process, in particular the
381
deployment of a membrane cascade purification system, could overcome the
382
existing bottleneck observed with the single-stage application to deliver
383
biomethane from biogas.
384
provided by the Széchenyi 2020 Programme under the project
EFOP-3.6.1-16-389
2016-00015, and by the Excellence of Strategic R+D Workshops under the
390
project GINOP-2.3.2-15 (which encompasses the development of modular,
391
mobile water treatment systems and wastewater treatment technologies based at
392
the University of Pannonia to enhance growing dynamic exportation from
393
Hungary between 2016 and 2020). The János Bolyai Research Scholarship of
394
the Hungarian Academy of Sciences is duly acknowledged for the support. This
395
work was supported by the Korea Research Fellowship Program through the
396
National Research Foundation of Korea (NRF) funded by the Ministry of Science
397
1
and ICT (Grant No: 2016H1D3A1908953).This work was supported by the New
398
& Renewable Energy Core Technology Program of the Korea Institute of Energy
399
Technology Evaluation and Planning (KETEP) granted financial resource from
400
the Ministry of Trade, Industry & Energy, Republic of Korea (No.
401
20173010092470).
402 403
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511 512
1
Figure legends
513 514
Fig. 1 – Image of the gas separation membrane system (left-hand side) with
515
the PI membrane module installed (right-hand side).
516
Fig. 3 – The effect of the splitting factor (R/F) on the methane concentration
520
on the retentate side (diamond) and CO2/CH4 permselectivity (square) using
521
the model biogas.
522
Fig. 4 – The effect of pF/pp on the methane concentration on the retentate
523
side (diamond) and CO2/CH4 permselectivity (square) using the real biogas.
524
Fig. 5 – The effect of the splitting factor (R/F) on the methane concentration
525
of the retentate side (diamond) and CO2/CH4 permselectivity (square) using
526
the real biogas.
527
Fig. 6 – The time dependency of the composition of the permeate under the
528
conditions listed in Table 3. Square: carbon dioxide; Diamond: methane;
529
Triangle: nitrogen.
530
Fig. 7 – The time dependency of the composition of the retentate under the
531
conditions listed in Table 3. Square: carbon dioxide; Diamond: methane;
532
Triangle: nitrogen.
533 534
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49
22
Table 1 – Experimental conditions and results using the binary gas mixture (80 vol.% CH4, 20 vol.% CO2)
pF
(bar) pF/pP (-) R/F
(-) Gas concentration (vol.%) J (dm3 min-1 bar-1 at STP) CO2/CH4
Permselectivity (-) Ymethane (%)
Permeate Retentate CH4 CO2
CH4 CO2 CH4 CO2
7.0 1.78 0.89 64.9 35.1 81.9 18.1 5.53 15.43 2.79 90.8
11.8 2.33 0.65 62.6 37.4 89.3 10.7 2.81 17.31 6.17 72.7
12.3 2.42 0.66 53.2 46.8 93.8 6.2 4.85 34.08 7.03 77.4
13.5 1.76 0.73 55.7 44.3 89.1 10.9 9.00 53.54 5.95 81.0
13.6 1.77 0.73 69.5 30.5 83.9 16.1 1.96 10.35 5.27 76.4
14.5 1.40 0.81 74.6 25.4 81.3 18.7 2.11 7.64 3.63 81.9
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49
23
Table 2 – Experimental conditions and results using the biogas mixture containing 70 vol.% CH4, 19.8 vol.% CO2, 9.2 vol.% N2 and unknown trace substances to balance.
pF
(bar) pF/pp (-) R/F (-) Gas concentration (vol.%) J (dm3 min-1 bar-1 at STP) CO2/CH4
Permselectivity (-) Ymethane (%)
Permeate Retentate CH4 CO2
CH4 CO2 N2 CH4 CO2 N2
8.5 1.36 0.78 69.4 28.5 2.2 72.3 17.2 10.1 8.74 33.92 3.88 80.9
7.7 1.43 0.79 69.2 19.9 10.0 70.2 19.7 9.5 7.66 7.84 1.04 79.1
4.3 2.65 0.66 49.3 42.8 6.9 80.7 7.5 11.4 5.26 46.58 8.85 76.0
6.4 1.76 0.93 58.5 31.7 8.8 70.8 18.3 10.2 2.52 8.89 3.53 94.3
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49
24
Table 3 – Average experimental conditions for the assessment of process stability during longer-term biogas (57.4 vol.%
CH4, 39 vol.% CO2, 2.5 vol.% N2 and unknown trace substances to balance) permeation conducted at 50 oC.
pF (bar) pF/pp (-) R/F (-) Gas concentration (vol.%) J (dm3 min-1 bar-1 at STP) CO2/CH4
Permselectivity (-) Ymethane (%)
Permeate Retentate CH4 CO2
CH4 CO2 N2 CH4 CO2 N2
10.8 5.48 0.58 21.6 75.8 1.4 81.7 14.6 2.9 1.07 12.55 11.77 82.9
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65
25 Fig. 1
1
Methane concentration in retentate (vol.%) CO2/CH4 selectivity (-)
pF/pP (-)
1
Methane concentration in retentate (vol.%) CO2/CH4 selectivity (-)
R/F (-)
1
Methane concentration in retentate (vol.%) CO2/CH4 selectivity (-)
pF/pP (-)
1
Methane concentration in retentate (vol.%) CO2/CH4 selectivity (-)
R/F (-)
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65
30 Fig. 6
0 10 20 30 40 50 60 70 80 90
0 1 2 3 4 5 6 7 8 9
Gas concentration in permeate (vol.%)
Time (h)
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65
31 Fig. 7
0 10 20 30 40 50 60 70 80 90
0 1 2 3 4 5 6 7 8 9
Gas concentration in retentate (vol.%)
Time (h)